Journal Article
Cation distribution in calcic amphiboles from the Nuggihalli Greenstone belt, S. India and its geological significance
Mitra, S; Bidyananda, M; ,
Neues Jahrbuch fuer Mineralogie. Abhandlungen
ISSN: 0077-7757
E SCHWEIZERBARTSCHE VERLAGSBUCHHANDLUNG
Amphiboles from Nuggihalli schist belt are analyzed and found to be mainly magnesio-hornblende, ferro-hornblende and actinolitic hornblende. Mossbauer study of actinolitic hornblende shows presence of iron in the M 1, M2, M3 and M4 sites. A lower QS value (1.66 mm/s) at M4 site for Fe 21 indicates a faster rate of crystallization of the studied sample whereas Fe3+/Fe2+ ratio (0.14) indicates that the amphiboles were formed in an oxidizing environment (high fO(2)). Based on plagioclase-amphibole geothermometry, equilibration temperature of meta-anorthosite is seen to range between 648degreesC (921 K) and 704degreesC (977K). The amphibolite is found to be equilibrated in a water rich environment (X-H2O ranging from 0.81 to 0.89). Considering the pressures as 5 and 7kbar, and the amphibolite subassemblages such as, calcite-rutile-quartz and epidote-plagioclase-calcite, in the T-X-CO2 section (derived from THERMOCALC program) the invariant points are seen to be at (i) X-CO2 = 0.11, T = 493 degreesC and (ii) X-CO2 = 0.19, T = 562 degreesC at the respective pressures. The calculated M2-M4 distribution coefficient (K-D) in Nuggihalli actinolitic hornblende is found to be 0.0034, which corresponds to a temperature of equilibration at around 435degreesC.
Physical Sciences - Other Topics; Greenstone belt, Dharwar craton, Nuggihalli, amphibole, Mossbauer; meta-anorthosite, amphibolite, geothermobarometry, assemblage,; THERMOCALC, equilibrium temperature; progressive metamorphism, mossbauer-spectroscopy, phase-relations,; hornblendes, tremolite, temperature, pressure, rocks, mineralogy,; chemistry